The spread spectrum communication (hereinbelow called SS communication) is a method, by which on the transmitter side, the primary modulation is effected by using a code called PN (pseudo-noise) code, which has a speed significantly higher than the information signal to be transmitted, and further the carrier wave is subjected to a secondary modulation (the order of the primary modulation and the secondary modulation can be reversed) to be transmitted, and on the receiver side, reverse spreading is effected by correlation detection between the PN code and the received information and the information signal is restored by synchronized detection, etc. thereof. In general, it is said that this method is strong against disturbance wave and selective fading. This reason will be explained below.
At first, on the characteristics against disturbance, in the case where narrow band disturbance wave is mixed in the received signal, as indicated in FIG. 14(A), in the signal spectrum obtained as the result of the correlation detection between this received signal and the PN code, the signal component is reversely spread into a bandwidth Bd and on the contrary the disturbance wave is spread into a bandwidth Bc, as indicated in FIG. 14(B). When this output signal is made to pass through a band pass filter having the bandwidth the, electric power of the disturbance wave is reduced to Bd/Bc, as indicated in FIG. 14(C) (this is called processing gain). That is, it can be said that the resistance against the disturbance wave is increased, corresponding to this processing gain. The disturbance wave becomes white-noise-like, i.e. non-understandable noise, by the correlation detection.
Next, on the characteristics against selective fading, in the case where the selective fading is produced by multipath interference, in the received signal, ripple is produced within the bandwidth Bc, as indicated in FIG. 15 and a part of the information of the signal is lost. However, since the information signal is spread so that a plurality of information signals are produced, information remains always in the whole and therefore, when it is reversely spread, the information signal can be restored. That is, it can be said that, contrarily to the narrow band communication, the resistance against the selective fading is obtained by spreading spectrum.
However, in the case where disturbance wave is mixed at a level exceeding the processing gain, as indicated in FIGS. 16(A), 16(B) and 16(C), the information signal is buried in noise also after the correlation detection and can be restored no more. The above description has been made under an assumption that the synchronization is established. In reality, with disturbance wave over a certain level below the processing gain the synchronization can not be established and thus information can be restored no more.
Further, depending on the interval (frequency) between ripples due to the selective fading, the depth (attenuation) and the level of the received signal (expressed by the ratio to thermal noise), even if the reverse spreading is effected by the correlation detection, the information signal is buried in the thermal noise and cannot be restored. That is, even if the spread information signal is below the level of the thermal noise under an environment, where there is no selective fading, as indicated in FIG. 17, the information signal can be restored by the reverse spreading. However, as indicated in FIG. 18, much information can be lost by a selective fading giving deep variations so that the information signal is buried in the thermal noise even by the reverse spreading and that it can be restored no more.
Consequently, by prior art methods, a method has been conceived in order to remove these drawbacks, by which similarly to the narrow band communication, the same information is transmitted, divided into a plurality of bands and a given band, on which influences of disturbance waves and selective fading are slight, is chosen.
For example, a plural transmission method is known (FIG. 6A), by which a PN code generating circuit 21, multipliers 23 and 24, local oscillators 25 and 26, and an adder 27 are used. FIG. 6B shows the frequency spectrum of the output thereof.
However, by this prior art method a plurality of carriers, multipliers, etc. are required at the transmitter and therefore this method had drawback that no cheap receiving device can be constructed. Further, since the number of parts increases, this method has a problem in the attempt to reduce the size.